Envelopes can have holes drilled in them. Hole drilling can
be accomplished offline after they have gone through the converting process,
or depending on the construction of the envelope, they may be punched online
as they are converted.

Examples:

Holes are punched through an inter-office type envelope
which allows the user to see if the envelope contains any material.

On a clasp envelope the flap is punched to allow
the clasp to be fed through the flat to seal the envelope. A patch
washer is sometimes applied first and then the hole is punched through
the washer. The patch washer is used to reinforce the hole.

A hole may be punched on an envelope that needs to be hung on a hook.
A hole used for this purpose is sometimes reinforced with a metal
eyelet.